Frequency locking of multisection laser diodes

ABSTRACT

A multi-section laser diode control system comprising a multi-section laser diode ( 10 ), microprocessor controller ( 24 ), digital-to-analogue converter ( 28 ), driver circuit ( 30 ) and wavelength locker ( 14 ) is modified by inclusion of a locking circuit ( 40 ) which generates an analogue correction signal . . . 1 V ph responsive to measurements of the laser output made by the wavelength locker. The analogue correction signal is added to the preset phase voltage V ph asserted by the microprocessor controller to provide fast feedback that bypasses the microprocessor controller. This novel feedback is made possible by avoiding the use of the standard prior art control algorithm which requires a division calculation to be performed. Instead, novel control algorithms based purely on additions, subtractions and multiplications are used. The laser can thus be locked to its target output frequency without having to place slow analogue-to-digital and digital-to-analogue converters in the feedback control path.

BACKGROUND OF THE INVENTION

The invention relates to a multi-section laser diode that can beswitched between different wavelengths, more especially to a lasersystem comprising a control circuit that enables a multi-section laserdiode to be switched rapidly between different wavelengths.

The original multisection diode laser is a three-section tunabledistributed Bragg reflector (DBR) laser. Other types of multisectiondiode lasers are the sampled grating DBR (SG-DBR) and the superstructuresampled DBR (SSG-DBR) which both have four sections. A furthermultisection diode laser is the grating-assisted coupler with rearsampled or superstructure grating reflector (GCSR), which also has foursections. A review of such lasers is given in reference [1].

FIG. 1 is a basic schematic drawing of a SG-DBR 10. The laser comprisesback and front reflector sections 2 and 8 with an intervening gain oractive section 6 and phase section 4. An antireflection coating 9 isusually provided on the front and/or rear facet of the chip to avoidfacet modes. The back and front reflectors take the form of sampledBragg gratings 3 and 5. The pitch of the gratings of the back and frontreflectors is slightly different, to provide a Vernier tuning effectthrough varying the current supplied to these sections. The optical pathlength of the cavity can also be tuned with the phase section, forexample by refractive index changes induced by varying the carrierdensity in this section. A more detailed description of the SG-DBR andother tunable multi-section diode lasers can be found elsewhere [1].

Multisection diode lasers are useful in wavelength division multiplexed(WDM) systems. Example applications are as transmitter sources, aswavelength converters in optical cross connects (OXCs) and for referencesources in heterodyne receivers. Typically, WDM systems have channelspacings conforming to the International Telecommunications Union (ITU)standard G692, which has a fixed point at 193.1 THz and inter-channelspacings at an integer multiple of 50 GHz or 100 GHz. An example denseWDM (DWDM) system could have a 50 GHz channel spacing and range from 191THz to 196 THz (1525-1560 nm).

The raison d'être of multisection diode lasers is their wavelengthtunability. Each section of the laser diode is supplied with a drivecurrent, and the lasing wavelength is a function of the set of drivecurrents, this function generally being quite complex. Setting theoutput wavelength of such a laser is thus usually performed by asophisticated microprocessor controlled control system. As well as thefact that there is a complex relation between output wavelength and theset of drive currents, there is the additional factor that wavelengthswitching of the laser destroys its thermal equilibrium, which resultsin transient wavelength instabilities until thermal equilibrium isreached at the new set of drive currents. The time needed fortemperature stabilisation can be quite long.

The transient thermal properties consist of two main effects.

A first effect is that, directly after the laser is switched, thethermal gradient across the device to the heat sink upon which it ismounted will be different to that measured at steady state operatingconditions for these currents, due to a different heating levelgenerated in the laser as the currents are different. This steady statetemperature gradient will reassert itself over a period measured in atimescale from a few hundred nanoseconds to tens of microseconds.Because the device is at a different temperature during this period sometemperature tuning of the wavelength occurs. For a positive (negative)change in tuning current the change in temperature will be such that thedevice is initially colder (hotter) than at equilibrium for thosecurrents and some time will pass before the extra current dissipatesenough heat energy to change this. During that period the device will becolder (hotter) than expected so a blue (red) shift from the expectedoutput wavelength will occur.

A second effect takes place over a much longer timescale. The laser isthermally connected to a heat sink of finite thermal mass which has atemperature controller maintaining its temperature. The temperaturecontroller cannot react instantaneously to a change in temperature,which means that with an increase (decrease) in bias current, the heatsink will heat up (down). This in turn means that for a giventemperature gradient the device will have a different temperature,because the temperature the gradient is referenced from will bedifferent. This temperature change results in the temperature of thedevice overshooting and going higher (lower) than would be normal forthose currents. This effect will persist until the temperaturecontroller returns the heat sink to its normal temperature, which maytake 1-1.5 seconds.

To overcome the problems associated with transient (and non-transient)thermal effects, and also any other effects that cause the wavelength todeviate from the intended wavelength for a predetermined set of drivecurrents, a wavelength measuring system can be included which suppliesmeasurements of the output wavelength to the control system. The laserdrive current can then be adjusted in a feedback loop to provide lockingof the output to the desired output wavelength.

FIG. 2 shows a typical application example where a SG-DBR laser is usedas a source for a WDM system, with a microprocessor control system beingprovided for wavelength locking.

A SG-DBR 10 has a pigtailed output connection to an optical fibre 20. Anoptical coupler 12 is arranged in the optical fibre output path 20 tocouple off a small proportion of the output power, for example 5%. Thecoupler 12 may be a fused taper coupler, for example. The part of theoutput beam diverted off by the coupler 12 is supplied to an opticalwavelength locker 14, for example a JDS Uniphase WL5000 SeriesWavelength Locker. The optical wavelength locker 14 is a wavelengthmeasuring device based on a Fabry-Perot etalon. For WDM applications,the etalon is designed to have its cyclical frequency response matchedto the ITU grid.

FIG. 3 shows the frequency response of the etalon in terms of itspercentage throughput T as a function of frequency f. The frequencyresponse of the etalon is such that an ITU channel frequency occurs onthe maximum positive slope of the etalon peaks, as indicated in thefigure. The optical wavelength locker 14 includes first and secondphotodiodes PD1 and PD2. Photodiode PD1 is arranged to receive lighttransmitted by the etalon. Accordingly, with reference to FIG. 3, if theoutput frequency of the laser is, for example, greater than the ITUfrequency, the photodiode PD1 will receive a higher incident power levelP1 than it would at the ITU channel frequency. Similarly, if the outputfrequency of the laser is below the ITU channel frequency, the power P1incident on the photodiode PD1 will be lower than the value it wouldhave if the laser output was at the ITU channel frequency. Thephotodiode PD1 thus outputs a voltage V_(pd1) that can be used as abasis for generating an error signal relating to the frequency deviationof the laser output from the ITU channel frequency.

The second photodiode PD2 of the optical wavelength locker is arrangedto measure the optical power input to the locker 14, thereby providing ameasure of the total output power of the laser in the form of ameasurement voltage V_(pd2). The measurement voltages V_(pd1) andV_(pd2) are supplied by respective signal lines 16 and 18 to ananalogue-to-digital converter (ADC) 22. The ADC 22 may for example have12 bit resolution. The ADC 22 supplies the digitised measurementvoltages V_(pd1) and V_(pd2) to a microprocessor 24 which may beconnected to ancillary computer equipment through an interface 26.

When initially setting the laser 10 to a given ITU channel frequency,the microprocessor 24 refers to a predetermined set of drive voltagesV_(f) V_(b) V_(g) and V_(ph) for the ITU channel frequency concerned.The sets of drive voltages may be conveniently held in a look-up table,for example. The microprocessor 24 may thus include on-chip memory forthis purpose, for example flash memory. To set the laser 10 to aparticular ITU channel frequency, the microprocessor 24 asserts a set ofvoltages to a digital-to-analogue converter (DAC) 28. The DAC 28 mayhave 12 bit resolution, for example. The DAC 28 then supplies thesevoltages to a driver circuit 30 which converts the voltages tocorresponding drive currents I_(f) I_(b) I_(g) and I_(ph) which are thenapplied to the front reflector, back reflector, gain and phase sections8, 2, 6 and 4 respectively of the SG-DBR 10.

Feedback from the optical wavelength locker 14 is provided in thiscontrol system by the microprocessor 24 continually re-adjusting the setof voltages sent to the DAC 28 on the basis of the measured voltagesV_(pd1) and V_(pd2). The feedback adjustment is implemented principallythrough varying I_(ph), the current applied to the phase section 4 ofthe SG-DBR 10. The manner in which this is performed is now described.First of all, however, it is noted that, although the active wavelengthcontrol of the laser 10 is effected primarily through adjusting thephase current, adjusting the phase current will generally have otherconsequential effects, such as causing changes in the cavity loss. Thesecan be compensated for by adjusting the gain current I_(g).(Alternatively, compensation may be achieved using an external variableoptical attenuator (VOA) arranged in the output path 20 after thecoupler 12.) Consequently, although the wavelength control isprincipally implemented by varying the phase current, the gain currentand possibly either of the other control currents may be changed as partof the feedback. For the sake of simplicity, the following descriptionrefers only to variance of the phase current.

The phase current I_(ph) is varied by a correction factor I_(err)defined by the following equation

$I_{err} = {k\left( {\frac{V_{{pd}\; 1}}{V_{{pd}\; 2}} - R_{ITU}} \right)}$where V_(pd1) and V_(pd2) are voltages proportional to the powers P1 andP2, as described above, R_(ITU) is the value of V_(pd1)/V_(pd2) at anITU channel frequency, and k is a constant factor. Generally a separatevalue for R_(ITU) will be used for each ITU channel, these values beingstored in a look-up table, which may form part of on-chip memory of thecontrolling microprocessor, or may be held in an EPROM or other memory.The values of R_(ITU) will typically be preset during a calibrationperformed at the manufacturing stage. Correction of the phase current,by setting V_(ph)→V_(ph)−V_(err) in each control cycle, is effectivesince the error current I_(err) is proportional to the wavelengthdeviation from the ITU channel wavelength. Thus, if the value of V_(err)is negative, the phase current is increased by a small amount, and viceversa. The procedure repeated until the different between the measuredvalue and the stored value is within a tolerance. The phase current isthus used to provide fine tuning of the output frequency of the laser,with increases in phase current typically causing increases in theoutput frequency of the laser.

A conventional control system for wavelength locking such as thatdescribed above, or in reference [2], is thus based on calculating anerror factor from the deviation of the ratio P1/P2 from a desired valueof P1/P2 for the wavelength channel concerned, stored as the compoundratio value R_(ITU).

The control loop is thus dependent on performing a division operation.Division operations can be easily performed using a microprocessor, suchas a digital signal processor (DSP), and can also be performed bycertain types of multiplier components. However, microprocessor andmultiplier chip implementations both have limitations.

A drawback of using DSP or other microprocessor chips is that ananalogue-to-digital (A-D) sample must be made at the input, and adigital-to-analogue (D-A) output must be made at the output. This takessome time to perform and limits the locking speed of the system.

A drawback of using multiplier chips is their accuracy and bandwidth.The accuracy is typically worse than ±2% and the bandwidth will belimited to a maximum of about 1 MHz. This limits the speed and accuracyof the locking mechanism.

With the prior art control system using microprocessor chips, or withmultiplier chips that allow divide operations, it should be possible toimprove the switching speed beyond the tens of millisecond range,perhaps up to as fast as tens of microseconds. However, at least withpresent commercially available electronic components, it is not possibleto attain faster switching times.

However, ideally, the control system should have a response timeapproaching the fundamental limit of the switching time of a diodelaser, which is of the order of ten nanoseconds.

SUMMARY OF THE INVENTION

According to the invention there is provided a way of locking amulti-section laser diode to a specified frequency that can beimplemented without performing a division, and thus allows the feedbackcontrol for wavelength locking to be performed purely with simpleelectronic components such as adders, subtractors and multipliers.

Specifically, a microprocessor is not present in the feedback path, sothat slow digital-to-analogue and analogue-to-digital conversions arenot part of the control loop. As a result very rapid wavelength lockingcan be achieved. This opens the way for using multi-section laser diodesfor applications which require rapid switching between frequencychannels. For example, a laser system according to the invention can beused for optical packet switching networks.

According to one aspect of the invention there is provided a systemcomprising: a multi-section diode laser comprising a plurality ofsections having respective control inputs; a memory storing a pluralityof sets of control input values, each set of control input valuescorresponding to a target output frequency of the laser; amicroprocessor controller operable to select one of the sets of controlinput values and assert it through a digital-to-analogue converter tosupply a corresponding set of analogue control signals to the laser viaits control inputs; and a locking circuit arranged to generate andoutput an analogue correction signal responsive to measurements of thelaser output, the analogue correction signal being combined with one ofthe analogue control signals prior to supply of that control signal tothe associated control input of the laser, thereby to lock the laser tothe target output frequency.

The microprocessor, with its slow ADC and DAC input/output delays isthus bypassed by a high speed feedback control path based on analoguesignals

In one embodiment, only one analogue correction signal is used. Inanother embodiment the locking circuit is arranged to generate andoutput a further analogue correction signal responsive to measurementsof the laser output, the further analogue correction signal beingcombined with a further one of the analogue control signals prior tosupply of that control signal to the associated control input of thelaser.

For the laser output measurements, there is provided in embodiments ofthe invention: a frequency selective element having a cyclical frequencyresponse matched to the wavelength channels and arranged to receive atleast a part of the laser output; a first detector operable to supplythe locking circuit with a first power value indicative of the powerbeing transmitted by the frequency selective element; and a seconddetector operable to supply the locking circuit with a second powervalue indicative of the total power being output by the laser.

According to another aspect of the invention, there is provided aprocess for controlling a laser comprising a plurality of sectionshaving respective control inputs for receiving respective analoguecontrol signals, the process comprising: storing in a memory a pluralityof sets of control input values, each set of control input valuescorresponding to a target output frequency of the laser; setting anoutput channel of the laser by using a microprocessor to assert one ofthe sets of control input values through an digital-to-analogueconverter and subsequent driver circuitry to generate a correspondingset of analogue control signals that are supplied to the control inputsof the laser; and locking the laser to the target output frequency bygenerating and outputting an analogue correction signal responsive tomeasurements of the laser output, the analogue correction signal beingcombined with one of the analogue control signals prior to supply ofthat control signal to the associated control input of the laser.

According to a first embodiment of the invention there is provided acontrol process for locking a laser to any one of a plurality of desiredwavelengths lying in respective wavelength channels, comprising:

-   (a) setting the laser to output within one of the wavelength    channels;-   (b) measuring a first power value indicative of the power being    transmitted by a frequency selective element having a cyclical    frequency response matched to the wavelength channels;-   (c) measuring a second power value indicative of the total power    being output by the laser;-   (d) determining a first error value from the difference between the    first power value and a desired first power value for the wavelength    channel currently set;-   (e) determining a second error value from the difference between the    second power value and a desired second power value for the    wavelength channel currently set; and-   (f) determining a laser control parameter from the difference    between one of the first and second error values and a constant    factor multiplied by the other of the first and second error values;-   (g) using the laser control parameter to lock the laser to the    desired wavelength.

In the first embodiment, the constant factor for the wavelength channelcurrently set may be equal to the first desired power value divided bythe second desired power value. The laser may have a phase section andthe laser control parameter is used to adjust a phase current suppliedto the phase section. The second error value may be used to determine afurther laser control parameter that is applied to adjust gain orattenuation in the laser or its output.

According to a second embodiment of the invention there is provided acontrol process for locking a laser to any one of a plurality of desiredwavelengths lying in respective wavelength channels, comprising:

-   (a) setting the laser to output within one of the wavelength    channels;-   (b) measuring a first power value indicative of the power being    output by the laser at the desired wavelength;-   (c) measuring a second power value indicative of the total power    being output by the laser;-   (d) determining a first error value from the difference between the    first power value and a desired first power value for the wavelength    channel currently set;-   (e) determining a second error value from the difference between the    second power value and a desired second power value for the    wavelength channel currently set; and-   (f) determining first and second laser control parameters from the    first and second error values respectively, and using the first and    second laser control parameters to adjust first and second control    inputs to the laser respectively.

In the second embodiment, the first control input may be a phase currentof a phase section of the laser. The second control input may be a gaincurrent applied to a gain section of the laser, or a control currentapplied to a variable attenuator or gain element arranged in the outputpath of the laser.

According to the first embodiment of the invention, there is alsoprovided a laser system comprising:

-   (a) a wavelength tunable laser source for providing a laser output    in any one of a plurality of wavelength channels;-   (c) a frequency selective element having a cyclical frequency    response matched to the wavelength channels and arranged to receive    at least a part of the laser output; and-   (d) a first detector operable to measure a first power value    indicative of the power being transmitted by the frequency selective    element;-   (e) a second detector operable to measure a second power value    indicative of the total power being output by the laser; and-   (b) a controller arranged to receive the first and second power    values from the first and second detectors, and operable in a    channel setting mode to set the laser source to output in any one of    the plurality of wavelength channels and in a wavelength locking    mode to lock the laser output to a desired wavelength in each    wavelength channel using feedback control, wherein the feedback    control comprises:-   (i) determining a first error value from the difference between the    first power value and a desired first power value for the wavelength    channel currently set;-   (ii) determining a second error value from the difference between    the second power value and a desired second power value for the    wavelength channel currently set; and-   (iii) determining a laser control parameter from the difference    between the first error value and a constant factor multiplied by    the second error value; and-   (iv) using the laser control parameter to output a control signal to    the laser so as to stabilise the laser output to the desired    wavelength.

Advantageously, the wavelength locking mode may be deactivated for aperiod during the channel setting mode, for example a period of 1-50nanoseconds, more preferably 5-30, still more preferably 10-20nanoseconds.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same maybe carried into effect reference is now made by way of example to theaccompanying drawings in which:

FIG. 1 shows a sampled grating distributed Bragg reflector (SG-DBR)laser diode, as known in the prior art;

FIG. 2 shows a SG-DBR with an associated feedback control systemincluding a wavelength locker, as known in the prior art;

FIG. 3 shows the percentage throughput T of a Fabry-Perot etalon of thewavelength locker as a function of frequency ƒ.

FIG. 4 shows a SG-DBR with an associated feedback control systemcomprising a locking circuit according to a first embodiment of theinvention;

FIG. 5 shows the locking circuit of FIG. 4 in more detail;

FIG. 6 shows a SG-DBR with an associated feedback control systemcomprising an alternative locking circuit according to a secondembodiment of the invention;

FIG. 7 shows the locking circuit of FIG. 6 in more detail;

FIG. 8 shows a distributed Bragg reflector (DBR) laser diode; and

FIG. 9 shows a grating-assisted coupler with rear sampled orsuperstructure grating reflector (GCSR) laser diode.

DETAILED DESCRIPTION

FIG. 4 shows a laser with an associated control system according to afirst embodiment of the invention. Many of the components will berecognised as being common to the prior art example illustrated in FIG.2 and described above. For clarity, the same reference numerals are usedto refer to like or comparable components.

A SG-DBR 10 is used as the laser source and has a pigtailed outputconnection to an optical fibre 20. A microprocessor 24, for example aDSP, is provided for setting the laser 10 to a given ITU channelfrequency (assuming a DWDM application). The microprocessor 24 may beconnected to ancillary computer equipment through an interface 26. Themicroprocessor 24 refers to a predetermined set of drive voltages V_(f)V_(b) V_(g) and V_(ph) for the ITU channel frequency concerned. The setsof drive voltages may be conveniently held in a look-up table, forexample. The microprocessor 24 may thus include on-chip memory for thispurpose, for example flash memory. Alternatively, remote memory such asEPROM may be used and accessed by the microprocessor 24 through theinterface 26. To set the laser 10 to a particular ITU channel frequency,the microprocessor 24 asserts a set of voltages to a digital-to-analogueconverter (DAC) 28. The DAC 28 may have 12 bit resolution, for example.The DAC 28 supplies the voltages V_(f) V_(b) V_(g) and V_(ph) to adriver circuit 30. In the case of the phase control, an adder 25 isarranged between the DAC 28 and the driver circuit 30, having as a firstinput the phase voltage V_(ph) from the DAC 28 and a correction voltageΔV_(ph) as a second input. The error correction of the phase voltage isdescribed in more detail further below. The driver circuit 30 convertsthe voltages to corresponding drive currents I_(f) I_(b) I_(g) andI_(ph) which are then applied to the front reflector, back reflector,gain and phase sections 8, 2, 6 and 4 respectively of the SG-DBR 10.

The DAC 28 can also be used to shape the pulse as the currents arechanged from one operating point to another. In this way the laser canswitch faster if an overshoot is provided.

In the output path 20 of the laser 10, an optical coupler 12 is arrangedto couple off a small proportion of the output power, for example 5%.The coupler 12 may be a fused taper coupler, for example. The part ofthe output beam diverted off by the coupler 12 is supplied to an opticalwavelength locker 14, for example a JDS Uniphase WL5000 SeriesWavelength Locker. The optical wavelength locker 14 is a wavelengthmeasuring device based on a Fabry-Perot etalon. (Alternatively, a longgrating may be used instead of the etalon.) For WDM applications, theetalon is designed to have its cyclical frequency response matched tothe ITU grid. The frequency response of the etalon is as alreadydescribed with reference to FIG. 3.

The optical wavelength locker 14 includes first and second photodiodesPD1 and PD2. Photodiode PD1 is arranged to receive light transmitted bythe etalon. Accordingly, with reference to FIG. 3, if the outputfrequency of the laser is greater than the ITU frequency, the photodiodePD1 will receive a higher incident power level P1 than it would at theITU channel frequency. Similarly, if the output frequency of the laseris below the ITU channel frequency, the power P1 incident on thephotodiode PD1 will be lower than the value it would have if the laseroutput was at the ITU channel frequency. (In an alternative design, thesigns will be reversed if the ITU channel frequencies are aligned withthe falling etalon flanks, instead of the rising flanks as illustratedin FIG. 3.) The photodiode PD1 thus outputs a voltage V_(pd1) that canbe used as a basis for generating an error signal relating to thefrequency deviation of the laser output from the ITU channel frequency.The second photodiode PD2 of the optical wavelength locker is arrangedto measure the optical power input to the locker 14, thereby providing ameasure of the total output power of the laser in the form of ameasurement voltage V_(pd2). The measurement voltages V_(pd1) andV_(pd2) are supplied by respective signal lines 16 and 18 to a lockingcircuit 40.

The purpose of the locking circuit 40 is to generate the correctionvoltage ΔV_(ph) which is added to the phase voltage V_(ph) generated bythe DAC 28 using the adder 25, so that the phase voltage supplied to thedriver circuit 30 is given by V_(ph)′=V_(ph)+ΔV_(ph).

FIG. 5 shows the locking circuit in more detail. The locking circuitreceives four input signals, the measured voltages V_(pd1) and V_(pd2),and two preset calibration voltages V_(spd1) and V_(spd2) which are thevalues that V_(pd1) and V_(pd2) should have when the laser is outputtingat the desired ITU channel frequency (or other target output frequency).The voltages V_(spd1) and V_(spd2) are supplied to the locking circuitby the microprocessor 24 via the DAC 28. The inputs V_(spd1) andV_(spd2) are specific to each ITU channel and are obtained duringfactory calibration of the system by setting the laser to output at eachITU channel frequency and measuring V_(pd1) and V_(pd2). Therefore, ifV_(pd1)=V_(spd1) and V_(pd2)=V_(spd2) then the laser is outputting atthe correct frequency and V_(ph)=V_(ph)′.

The locking circuit operates as follows. The voltages V_(pd2)−V_(spd2)are combined by subtraction in a first logic subtractor 41 to obtain adeviation value for V_(pd2) from its calibration value. The consequentresult V_(pd2)−V_(spd2) is then multiplied by a constant factor k in afirst logic multiplier 42. The factor k is obtained from measurements ofthe locker and corresponds to the value of the ratio V_(pd1)/V_(pd2)that is obtained at the ITU channel frequency concerned. The factor k isconstant and embedded in the electronics.

The voltages V_(pd1)−V_(spd1) are combined by subtraction in a secondlogic subtractor 43 to obtain a deviation value for V_(pd1) from itscalibration value. The other result k (V_(pd2)−V_(spd2)) is thensubtracted from V_(pd1)−V_(spd1) in a third logical subtractor 44. Thefactor k thus serves as a weighting factor when combining the twodeviations, to ensure that they are equally weighted.

Therefore, if the output power of the device changes, or the lockeralignment moves causing a change in the voltages received at V_(pd1) andV_(pd2), this can be normalised out without the need for a divide.

The combined result output from the third subtractor 44 is then scaledby a factor k′ using a second multiplier 45, so that the output signalfrom the second multiplier can be expressed byΔV _(ph) =k′((V _(pd1) −V _(spd1))−k(V _(pd2) −V _(spd2)))where ΔV_(ph) is the voltage which is to be added to the precalibratedphase voltage V_(ph) supplied by the microprocessor 24. The factor k′ isa lumped parameter, which may be considered to represent theproportional term in the feedback loop that determines the loop gain.

The logic elements and other circuit elements of the locking circuit maybe realised in conventional hardware, or with programmable logic, forexample in a field programmable gate array (FPGA).

While in practice the constant factor k is not exactly equal toV_(pd1)/V_(pd2) it will be close enough to make this term small. Forexample, if f=0.5 (indicating a 3 dB drop in the output power of thedevice) and the mismatch between k and the actual V_(pd1)/V_(pd2) is 1%then there is an overall error of 0.5%. This would cause a drift in theoutput frequency of the laser of <200 MHz which is well within a typicalspecification limit of +/−2.5 GHz.

The reason that k is not exactly equal to V_(pd1)/V_(pd2) is that thisratio will change slightly for different ITU wavelengths, typically witha 1% spread over a few tens of manometres around a wavelength of 1.5microns.

In summary, the locking circuit 40 is able to provide a correctionfactor for the phase current using only analogue circuit elements, andwithout the need for a microprocessor. This is achieved by using acontrol algorithm based on additions, subtractions and multiplications,and no division. A fast feedback control loop is thus added thatbypasses the microprocessor used to set the laser drive currents. Thefeedback control is based on separately determining deviations of themeasured voltages V_(pd1) and V_(pd2) from stored target values of theseparameters V_(spd1) and V_(spd2). This differs from the standard priorart control algorithm that is based on determining a deviation from atarget ratio of these voltage values. The memory configuration will thusbe different from the prior art in that for each set of controlvoltages, V_(f) V_(b) V_(g) and V_(ph), for a given target outputfrequency, there will be stored two values V_(spd1) and V_(spd2),instead of a single compound value R_(ITU), whereV_(spd1)/V_(spd2)=R_(ITU).

Advantageously, the microprocessor 24 is able to switch the lockingcircuit 40 off and on. A control line 27 for this purpose is shown inFIG. 4 with a dashed line. Specifically, there is a preferred mode ofoperation in which the locking circuit is disabled during each frequencyswitching event. In other words, each time the microprocessor 24 assertsa new set of control voltages V_(f) V_(b) V_(g) and V_(ph) to the DAC28, it switches off the locking circuit 40 at the same time or shortlybeforehand, and switches the locking circuit 40 back on shortlythereafter, for example a few tens of nanoseconds after switching, e.g.10, 20, 30 or 40 nanoseconds. This allows the laser to attain roughstabilisation after switching prior to activation of the locking circuitfeedback. The delay in enabling the locking circuit after switchingallows the laser to switch the output wavelength and then equalise thecarrier effects, so that, when the locking is enabled, the laser outputwavelength is within the locking range of the system, i.e. within thetarget ITU channel (even if not close to the centre frequency of thechannel). The locking can then compensate for ageing effects and thermaleffects.

It is also possible to use a feedforward mechanism during the initialtransition, to ensure the laser jumps to a wavelength in the lockingrange of the device.

It will also be understood that the measurement voltages V_(pd1) andV_(pd2) may be additionally supplied to the microprocessor 24 through asuitable ADC (not shown), as shown in the prior art example of FIG. 2.The measurement voltages may then be used by the microprocessor 24 tomodify the output voltages V_(f) V_(b) V_(g) and V_(ph) supplied to theDAC 28, thereby providing additional feedback similar to that of theprior art, operating to correct slowly varying changes in the wavelengthhaving time constants in the micro- or millisecond range.

The error signal ΔV_(ph) output from the locking circuit 40 can also beused to generate a global system error. If the value of the correctionvoltage exceeds a certain value, it can be inferred that the feedbackcontrol is not operating correctly, from which it can be furtherinferred that the system has suffered a global failure. For example, thelaser may not be outputting in the correct wavelength range or withstable power at a suitable level. In one implementation, a global errorsignal and system shutdown can be generated by supplying the errorsignal from the locking circuit to the microprocessor 24 via a suitableADC (not shown) or logic signal.

FIG. 6 shows a laser with an associated control system according to asecond embodiment of the invention. Many of the components will berecognised as being common to the first embodiment, namely a SG-DBRlaser source 10 with a pigtailed output connection to an optical fibre20, a microprocessor 24 having an associated interface 26 and beingconnected to control the laser 10 through a DAC 28 and driver circuit30, with feedback being provided by a wavelength locker 14 and lockingcircuit 140. For the sake of brevity, the second embodiment will bedescribed in terms of its similarities to and differences from the firstembodiment. As in the first embodiment, the second embodiment uses alocking circuit to provide high speed feedback for wavelength lockingthat bypasses the microprocessor 24 and that is based on four inputsignals, namely the measurement signals V_(pd1) and V_(pd2), and thecalibration signals V_(spd1) and V_(spd2). These voltages havingidentical significance and origin to those of the first embodiment. Thesecond embodiment differs from the first embodiment by virtue of theinternal design of the locking circuit, and the fact that the lockingcircuit provides two outputs, one for correcting the phase voltageΔV_(ph) (as in the first embodiment) and another for correcting the gainvoltage ΔV_(g) which are added to the microprocessor values for thephase and gain voltages by respective adders 25 and 23 interposedbetween the DAC 28 and driver circuit 30, as illustrated so that thedriver circuit receives gain and phase voltages V_(g)′ and V_(ph)′respectively, where V_(ph)′=V_(ph)+ΔV_(ph) and V_(g)′=V_(g)+ΔV_(g).

FIG. 7 shows the internal structure of the locking circuit 140 of thesecond embodiment. The locking circuit operates as follows. The voltagesV_(pd)−V_(spd2), representing measured total output power of the laser,are combined by subtraction in a first logic subtractor 141 to obtain adeviation value of the total output power from its calibration value.The consequent result V_(pd2)−V_(spd2) is then multiplied by a constantfactor k″ in a first logic multiplier 142.

The voltages V_(pd1)−V_(spd1), representing power passed through theetalon, or other wavelength selective element, are combined bysubtraction in a second logic subtractor 143 to obtain a deviation valuefor V_(pd1) from its calibration value.

The deviation values associated with V_(pd1) and V_(pd2) represent thetwo voltage correction signals for phase and gain respectively, and aresupplied to separate outputs of the locking circuit to the adders 25 and23 respectively in order to correct the corresponding drive currentssupplied to the laser. As an alternative, the deviation signal for thegain could be supplied to a variable attenuator or optical amplifierarranged in the laser's output path, for example in series with theoutput fibre 20. Thus, as in the first embodiment, the second embodimentprovides rapid feedback for wavelength locking without the need to use amicroprocessor or other component for performing division.

It will be understood that the variations described in relation to thefirst embodiment may also be applied to the second embodiment.

It will also be understood that in further embodiments the SG-DBR may bereplaced with a SG-DBR, or with a DBR or GCSR, as shown in FIGS. 8 and 9respectively, or with any other diode laser with a phase section.

FIG. 8 is a basic schematic drawing of a DBR. The laser comprises a backreflector section 102 with a gain or active section 106 and a phasesection 104. An antireflection coating 100 is usually provided on therear facet of the chip to avoid facet modes. The optical path length ofthe cavity can also be tuned with the phase section 104.

FIG. 9 is a basic schematic drawing of a GCSR. The laser comprises aSG-DBR reflector section 110, a phase section 112, a coupler section1114, and a gain or active section 116. Two planar waveguides 118 and120 extend through the SG-DBR, phase and coupler sections, with only thelower waveguide 120 extending through the gain section.

Annex 1

In the following, it is shown that the control algorithm of the firstembodiment reduces to the same mathematical form as the standard priorart control algorithm based on determining deviation of V_(pd1)/V_(pd2)from V_(spd1)/V_(spd2), thereby providing proof that feedback controlaccording to the first embodiment is stable.

The phase current adjustment I_(err) is obtained in the first embodimentof the invention using the following equationI _(err) −k ₁((V _(pd2) −V _(spd2))−k ₂(V _(pd1) −V _(spd1)))  eq. 1where

-   V_(pd2)=The voltage on the photodetector PD2 (Etalon power from    Locker)-   V_(spd2)=The set value from the DAC SPD2-   V_(pd1)=The voltage on the photodetector PD1 (Direct power from    Locker)-   V_(spd1)=The set voltage from the DAC SPD1-   I_(err)=The current to be added to the phase section of the laser    and where k₁, k₂ are constants

Locally to an ITU channel the following equation is trueV _(pd2)=(m ₁ λ+C ₁)P ₀andV_(pd1)=aP₀where P₀ is the output light power from the laser and a, m₁, C₁ areconstants.

In operation V_(spd2) and V_(spd1) are chosen to be equal to V_(pd2) andV_(pd1) while the laser is at an ITU channel, thereforeV _(spd2)=(m ₁λ_(ITU) +C ₁)P _(ITU)andV_(spd1)=aP_(ITU)

The locking mechanism adjusts the wavelength of the laser to the ITUchannel if the laser is not already there, so next we assume that theoutput power and the wavelength of the laser are not at the correctlevels, thereforeV _(pd2)=(m ₁(λ_(ITU)+Δλ)+C ₁)(P _(ITU) +ΔP)andV _(pd1) =a(P _(ITU) +ΔP)

Substituting these into eq. 1. givesI _(eer) =k ₁(((m ₁(λ_(ITU)+Δλ)+C ₁)(P _(ITU) +ΔP)−(m ₁λ_(ITU) +C ₁)P_(ITU))−k ₂(a(P _(ITU) +ΔP)−aP _(ITU)))

Which reduces toI _(err) =k ₁((m ₁λ_(ITU) ΔP+m ₁ ΔλP _(ITU) +m ₁ ΔλΔP+C ₁ ΔP)−k ₂(aΔP))

If k₂ is chosen to be as below

$k_{2} = {\frac{V_{{spd}\; 2}}{V_{{spd}\; 1}} = \frac{{m_{1}\lambda_{ITU}} + C_{1}}{a}}$it can be found thatI _(err) =k ₁(m ₁ ΔλP _(ITU) +m ₁ ΔλΔP) where m₁ΔλΔP→0I_(err)=k₁(m₁ΔλP_(ITU))therefore as k₁, m₁, P_(ITU) are all constants I_(err) is directlyproportional to the change in wavelength from the desired set point.

By contrast, the conventional technique is to use the following equation

$I_{err} = {k\left( {\frac{V_{{pd}\; 1}}{V_{{pd}\; 2}} - R_{ITU}} \right)}$where V_(pd1) and V_(pd2) are as before and R_(ITU) is the value ofV_(pd1)/V_(pd2) at an ITU channel. Therefore

$I_{err} = {k\left( {\frac{a\left( {P_{ITU} + {\Delta\; P}} \right)}{\left( {{m_{2}\left( {\lambda_{ITU} + {\Delta\;\lambda}} \right)} + C_{2}} \right)\left( {P_{ITU} + {\Delta\; P}} \right)} - R_{ITU}} \right)}$$R_{ITU} = \frac{a\; P_{ITU}}{{m_{2}\lambda_{ITU}} + C_{2}}$

It can be shown that in the region of an ITU channel by using a TaylorExpansion

${\Delta\; V_{{pd}\; 2}} \propto {{- \frac{1}{\Delta\; V_{{pd}\; 2}}}\mspace{14mu}{where}\mspace{14mu}\Delta\; V_{{pd}\; 2}} ⪡ V_{{pd}\; 2}$Therefore$V_{{pd}\; 2} = {{\frac{P}{{- {m_{3}\left( {\lambda_{ITU} + {\Delta\;\lambda}} \right)}} + C_{3}}\mspace{14mu}{where}\mspace{14mu}\Delta\;\lambda} ⪡ \lambda_{ITU}}$i.e. in the region of an ITU channel. This is true if the loop gain ofthe system is high, and hence

$I_{err} = {k\left( {\frac{{a\left( {P_{ITU} + {\Delta\; P}} \right)}\left( {{- {m_{3}\left( {\lambda_{ITU} + {\Delta\lambda}} \right)}} + C_{3}} \right)}{\left( {P_{ITU} + {\Delta\; P}} \right)} - \frac{a\;{P_{ITU}\left( {{{- m_{3}}\lambda_{ITU}} + C_{3}} \right)}}{P_{ITU}}} \right)}$I_(err) = −k(a m₃Δ λ)

Therefore by selecting the correct relationship between k and k₂,I_(err) is the same in both methods, the important difference being thatin the method of the invention computation of I_(err) does not require adivision operation. It is noted that there is a P term in the method ofthe first embodiment of the invention, but this can be made constant byusing gain equalisation in the laser so that the laser has the sameoutput power for all channels. Moreover, it will be understood thatI_(err) corresponds to the current change induced by changing the phasevoltage V_(ph) by the correction increment ΔV_(ph) in the description ofthe first embodiment.

REFERENCES

-   1. Chapter 2 of PhD by Geert Sarlet, University of Gent, Belgium    (September 2000) “Tunable laser diodes for WDM communication—Methods    for control and characterisation”-   2. WO-A-0049693 (Andersson)

1. A control process for locking a laser to any one of a plurality ofdesired wavelengths lying in respective wavelength channels, comprising:a. setting the laser to output within one of the wavelength channels; b.measuring a first power value indicative of the power being transmittedby a frequency selective element having a frequency response matched tothe wavelength channels; c. measuring a second power value indicative ofthe total power being output by the laser; d. determining a first errorvalue from the difference between the first power value and a desiredfirst power value for the wavelength channel currently set; e.determining a second error value from the difference between the secondpower value and a desired second power value for the wavelength channelcurrently set; f. determining a laser control parameter from thedifference between one of the first and second error values and aconstant factor multiplied by the other of the first and second errorvalues; and g. using the laser control parameter to lock the laser tothe desired wavelength.
 2. The control process according to claim 1,wherein the constant factor for the wavelength channel currently set isequal to the first desired power value divided by the second desiredpower value.
 3. The control process according to claim 1, wherein thelaser has a phase section and the laser control parameter is used toadjust a phase current supplied to the phase section.
 4. The controlprocess according to claim 1, wherein the second error value is used todetermine a further laser control parameter that is applied to adjustgain or attenuation in the laser or its output.
 5. The control processaccording to claim 1, wherein the laser control parameter is phasecurrent of a phase section of the laser.
 6. The control processaccording to claim 1, wherein the laser control parameter is a gaincurrent applied to a gain section of the laser, or a control currentapplied to a variable attenuator or gain element arranged in the outputpath of the laser.
 7. A laser system comprising: (a) a wavelengthtunable laser source for providing a laser output in any one of aplurality of wavelength channels; (b) a frequency selective elementhaving a frequency response matched to the wavelength channels andarranged to receive at least a part of the laser output; and (c) a firstdetector operable to measure a first power value indicative of the powerbeing transmitted by the frequency selective element; (d) a seconddetector operable to measure a second power value indicative of thetotal power being output by the laser; and (e) a controller arranged toreceive the first and second power values from the first and seconddetectors, and operable in a channel setting mode to set the lasersource to output in any one of the plurality of wavelength channels andin a wavelength locking mode to lock the laser output to a desiredwavelength in each wavelength channel using feedback control, whereinthe feedback control comprises: (i) determining a first error value fromthe difference between the first power value and a desired first powervalue for the wavelength channel currently set; (ii) determining asecond error value from the difference between the second power valueand a desired second power value for the wavelength channel currentlyset; (iii) determining a laser control parameter from the differencebetween the first error value and a constant factor multiplied by thesecond error value; and (iv) using the laser control parameter to outputa control signal to the laser so as to stablilise the laser output tothe desired wavelength.
 8. The laser system according to claim 7,wherein: the first detector is operable to supply a locking circuit withthe first power value indicative of the power being transmitted by thefrequency selective element; and the second detector is operable tosupply the locking circuit with the second power value indicative of thetotal power being output by the laser.
 9. The laser system according toclaim 7, wherein the laser source comprises a phase section and thecontrol parameter is a phase current supplied to the phase section. 10.The laser system according to claim 7, wherein the wavelength lockingmode is deactivated for a period during the channel setting mode.